There are relatively few mineral commodities that are suitable, by virtue of high melting point, mineral stability and physical and chemical properties, for use in refractory material applications above 1000.degree. C. Alumina (Al.sub.2 O.sub.3) easily satisfies all of these requirements and, due to its excellent chemical inertness, finds applications in many diverse temperatures and environments. For example, alumina refractories containing from 30 to 99 percent by weight Al.sub.2 O.sub.3 are widely used in high temperature processing and production operations, such as high temperature metallurgical, glass and cement processes.
In most all alumina refractories the bulk of the non-Al.sub.2 O.sub.3 material is silica (SiO.sub.2). Alumina also, typically, includes a number of impurities, such as free iron, titania and alkalies. All of the non-Al.sub.2 O.sub.3 constituents reduce the refractoriness of Al.sub.2 O.sub.3 by forming low temperature melting phases or glasses which limit the usefulness of the alumina. The use of alumina refractories is also limited, at times, due to contact with other external materials such as glasses or metallurgical slags. Generally, as the Al.sub.2 O.sub.2 content increases, there is a corresponding decrease in associated detrimental oxides and other impurities. Therefore, as the intended operating temperature of the alumina refractory increases, or as the operating atmosphere becomes more reactive, higher Al.sub.2 O.sub.3 content refractories must be used in order to retain the desired refractory properties and to minimize the detrimental effects caused by phase or glass formation by the non-Al.sub.2 O.sub.3 constituents. For example, at high temperatures there is a reaction between SiO.sub.2 and Al.sub.2 O.sub.3 resulting in the formation of mullite (3Al.sub.2 O.sub.3.2SiO.sub.2), which has a melting point of 1850.degree. C. As the Al.sub.2 O.sub.3 content drops below 70%, there is more and more unreacted, i.e., excess, SiO.sub.2, which may be present in the form of alpha quartz, cristobalite, or combined with impurities to form an amorphous glassy phase. If present as a glassy phase it can limit the high temperature properties of the refractory. If present as cristobalite, or even as quartz which is convertible to cristobalite above 1200.degree. C. in the presence of mineralizers, the cristobalite undergoes a low temperature inversion, accompanied by a 1% linear expansion, which results in poor spall resistance. The other impurities form low-melting secondary crystalline phases or glass phases which soften at high temperatures, causing deformation or loss of strength.
It would, of course, be desirable to utilize alumina refractories containing no non-Al.sub.2 O.sub.3 impurities. However, small quantities of these impurities are found in all but the most expensive refractory products and, unless very high Al.sub.2 O.sub.3 content refractory grade bauxite (greater than 85% Al.sub.2 O.sub.3) is readily available, the presence of excess silica will always be a problem. Although the world's reserves of refractory grade bauxite are extensive, due to politico-economic considerations, these reserves may not always be available to the United States. Therefore, an effort has been made in the United States to enhance domestic self sufficiency and reduce the United States dependence on imported refractory grade bauxite by trying to locate domestic mineral deposits of refractory grade bauxite, chemically beneficating clays and other Al.sub.2 O.sub.3 containing domestic resources to obtain high-Al.sub.2 O.sub.3 concentrate, and recycling high-Al.sub.2 O.sub.3 refractories. Another approach is to immobilize the impurities, e.g., tie them up by reaction, to prevent their formation of amorphous, glassy phases at grain boundaries and/or to stabilize the excess silica to prevent cristobalite formation in order to improve the high temperature properties of lower content Al.sub.2 O.sub.3 (42-70% Al.sub.2 O.sub.3) refractories produced from readily available domestic resources to the point where they could be substituted for the higher-Al.sub.2 O.sub.3 content refractories.
One attempt to improve the high temperature properties of alumina and other refractories is disclosed in U.S. Pat. No. 3,192,058 to Davies et al wherein a water insoluble, high purity oxide of chromium, particularly Cr.sub.2 O.sub.3, in finely divided particulate form is added to a refractory metal oxide aggegrate, such as alumina, during refractory batch forming. Although properties such as density, high temperature dimensional stability and slag resistance are purportedly increased, the method suffers from the need for grinding, sizing, batching and homogeneous mixing of particulate materials and for treating the refractory at the aggregate stage, rather than at desired stages after it is at least preliminarily formed. In U.S. Pat. Nos. 3,734,767 and 3,789,096 to Church et al a ceramic treatment process is disclosed for enhancing the hardness of a porous alumina refractory oxide base material by impregnating the porous body with a water soluble metal salt which is convertible to an oxide by low temperature curing, normally at temperatures less than 1000.degree. F. The purpose of the Church et al process is to form a new microcrystalline structure or a very close bond between the added oxide and the refractory oxide base material for providing the desired high hardness. According to Church et al the process is desirable for use on a substantially pure alumina, 85-90% Al.sub.2 O.sub.3, and is not at all suitable on silica. Therefore, the Church et al process would not be useful on a less than 70% Al.sub.2 O.sub.3 refractory, since such is not substantially pure and contains substantial amounts of silica. Moreover, there is no teaching in Church et al of how to immobilize or stabilize the silica, titania, iron or alkali impurities which form the high temperature deleterious phases. Rather, the object of Church et al is merely to add metal oxides to the alumina structure, as is evidenced by the low temperature curing which assures that the oxides do not further react with the alumina impurities.
It is, therefore, the purpose of the present invention to provide a simple, efficient, effective and relatively low cost method for improving the high temperature properties of relatively low-Al.sub.2 O.sub.3 content alumina refractories.